I shot the 338-378 Weatherby for years. It was a custom rifle with a 40" barrel and I used 300g SMK's. My load was 106g of Retumbo with a Fed 215 primer. This was a winter time load and was on the verge of pressure. With the 250g I would start around 95g of Retumbo and work your way up. Be careful and watch for pressure signs.

I have a couple of rifles chambered in 338 Kahn, which is an improved 338-378. The fireformed Kahn case has a little more capacity, but my load data should still work if you start at the low end and work up.

At 109 the bolt was stiff, so it was definatly OP. I have only done one round of testing and fired one round of each load. I really think 106, maybe 107 will be the max load in this rifle. This rifle is built on a Wby action with a 27" Lilja barrel.

I think with the stock 338-378 case you should be fine to start out at 102 grains of Retumbo and work up from there. I was using Federal 215M primers.

The load I have shot for several years is RL 25 with 250 grain Match Kings.

I would suggest you start out at around 102 or 103 and work up from there. This rifle has a 30" barrel and a Nesika action, so I don't think you will be able to get the same speeds because of the longer barrel and the fact the Nesika is a much stronger action than the WBY. Start at the low end. Your rifle may take the 108 grain load but I would certainly want to start much lower than that.

338-378 lineman,
Welcome to the forum.I have used a 338-378 accumark 30"tube for years and love it.I have found any of the slower burning powers works great in this rifle,and 250gr. bullets as well. There or mine anyway just isn't picky. my pet load for the 250 smk is 102 RL-22 @ around 3000fps. Although with the exception of win. failsafes it will shoot any 250gr bullet w/102gr rl-22 same poi . I plan on building a new one with no weatherby free bore. Use your chronograph watch for pressure signs,have fun and good luck.
Wayne